@dgerety I'm guessing the box that brings in the fiber also has a router built-in?  If so, you'll need to disable any routing functions,like DHCP, wireless, firewall and so on on their router so the Deco is the only device routing traffic. Do an internet search for using the model of the fiber box with my own router.  Hopefully someone else has already been down that road and wrote step-by-step instructions....

  @BradLouKy 

Thank you, Brad. I believe you nailed it. The installer never mentioned that the Nokia box is a router. To test it, I switched my Mac from my Deco to the Nokia wi-fi and got a whopping 64mbps. So I'll call FiberFirst and ask how to disable it. Thanks again. 

  @dgerety I called FiberFirst support, which was very helpful. The Nokia is a router. I just unplugged it and moved the Ethernet from it to the modem. However, it hasn't helped. I'm still getting about 150mbps on the Mini but over 500 on the iPhone and iPad. I haven't made any changes at all to the Deco system. I've run all of the Apple diagnostics I can find and they all show all is normal. The Apple Wireless Diagnostics generated a report with about 30 files, all Greek to me. At this point, I think I may make an appointment at the local Apple store for a weekday. 

  @dgerety The plot thickens, or maybe thins. I've been running all the speed tests using Safari, as I don't like Chrome. But out of curiosity, I just ran it on Chrome. 

 DOWNLOAD Mbps 840.68,  UPLOAD Mbps 552.17

Needless to say, I'm delighted, and I'll use Chrome if I have to, but there must be something askew in Safari settings. Any ideas?

Thanks again.